You're heating up a bag of popcorn, and suddenly — BAM BAM BAM — the kernels are exploding like tiny fireworks. What's going on in there?

Every popcorn kernel is like a tiny pressure cooker. Inside that hard shell is a drop of water and some starch. The shell is the secret: it's tough enough to trap everything inside, even when things get wild.

When you heat the kernel, the water inside starts to boil and turn into steam. Steam takes up way more space than water — about 20 times more. The steam pushes against the shell, harder and harder, but the shell won't give.

The pressure builds and builds. The starch inside gets hot and gooey, like melted candy. The kernel is now a tiny bomb, holding everything in. The temperature climbs to about 350 degrees Fahrenheit.

Finally, the shell can't take it anymore. It cracks — and BOOM! The pressurized steam explodes outward in a fraction of a second. The gooey starch flies out with it, instantly puffing up into foam.

The hot foam cools almost immediately in the air, turning into the light, crispy fluff we recognize. That's why popcorn is so full of air — it's frozen foam, like edible bubble wrap.

Kernels that don't pop — the ones at the bottom of the bowl — are usually damaged. A crack in the shell lets the steam leak out before pressure builds high enough. No pressure, no pop.

So the next time you hear that rat-a-tat-tat in the microwave, you'll know: it's not magic. It's tiny pressure bombs, each one waiting for the moment its shell finally says, "I give up."